Planar Quantum Dots Research Articles

ZnSe‐based laser diodes: New approaches

In this paper we report on recent results concerning conventional edge emitting laser structures containing either quantum wells or quantum dots as region and vertical-cavity surface-emitting lasers. In the first part a series of four similar laser structures containing quantum wells with an emission wavelength of 520 nm was grown by molecular beam epitaxy in order to perform a systematic study of lifetime improvement. They differed in the alternating implementation of an additional 5 nm thick ZnSSe layer with a high sulfur composition of 25% neighboring the quantum well. A high stability of the CdZnSSe active layer was observed by introducing such a kind of strain compensating layers. Lifetime measurements showed a significant improvement up to one order of magnitude using p- and n- & p-side layers. In the second part electro-optical characteristics of ridge and planar CdSe quantum dot laser diodes were compared. A reduction of the threshold current density by a factor of 4.7 for the ridge structure was obtained. This has to be associated to the reduction of current spreading inside the laser diodes. Furthermore, a significant slower degradation of CdSe quantum dot structures compared to common ZnSe-based QW structures was observed. An operating time over 2700 h in pulsed mode experiments at 50 A/cm2 in LED mode was achieved. In the third part we report on the realization of an optically pumped monolithic vertical-cavity surface-emitting laser operating at a wavelength of 511 nm. The microresonator has a quality factor of 3200 while the threshold excitation power density for the onset of lasing is 22 kW/cm2 at room temperature. Micropillars of different diameter fabricated out of this structure show discrete optical modes due to the three dimensional optical confinement of the optical wave. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrostatic cross-talk between quantum dot and quantum point contact charge read-out in few-electron quantum dot circuits

We investigate by numerical simulation the effect of electrostatic coupling between a quantum point contact (QPC) detector of various geometries and a planar double quantum dot in a few-electron quantum dot circuit. Our simulation is based on solving coupled Kohn-Sham and Poisson equations self-consistently by the finite element method. We use the Slater formula to determine the first electron charging bias point of the plunger gate, and give the corresponding variation of the QPC conductance as a function of the QPC gate bias at that particular point, which we interpret as the detector sensitivity. For all QPC designs, we show that the charging bias point of the plunger gate decreases with increasing QPC gate bias. This effect is enhanced for designs wherein large QPC detection sensitivity is found.

Temperature dependence of the electron distribution in a GaAs matrix with embedded InAs quantum dots

We report on the effect of the Debye averaging process on the CV characteristics of a sample containing four coupled planes of InAs self-assembled quantum dots. The observed electron distribution presented a dynamical dependence of the temperature during the C–V measurements which was explained in terms of the screening length dependence on the temperature. In addition, using the C–V data, we calculated the electron density at the planes containing the InAs dots and we have observed a high-temperature stability: the electron density at the quantum dots remained constant over a large range of temperature.

Nontrivial carrier recombination dynamics and optical properties of over‐excited GaN/AlN quantum dots

AbstractWe analyze the low temperature photoluminescence properties of two multi‐layer stacking of GaN/AlN quantum dots. We report drastic differences of linewidths between continuous wave and time‐resolved photoluminescence experiments. After the pulsed excitation, time‐resolved photoluminescence reveals a substantial red shift of the line, which keeps a fairly constant width. In continuous wave experiments, the screening of internal electric fields by accumulation of e–h pairs in quantum dot planes induces a blue‐shift. For samples with large number of quantum dot planes an unexpected narrowing of the emission line is observed when the laser intensity is increased. We assign the observed behaviors of energies and linewidths to the contributions of the in‐depth decrease of the degree of excitation of the different planes. Our interpretation is supported by the use of a model based on a self‐consistent solution of the Schrödinger and Poisson equations within the envelope function approximation. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral conductivity in GaAs/InAs quantum dot structures

Lateral conductivity effects have been investigated in self-organised InAs quantum dot (QD) structures grown in a GaAs matrix with different cap layers. Current-voltage (I-V), capacitance-voltage (C-V), DLTS, capacitance and conductance frequency dependence, fast defect transient (FDT), and electron beam induced conductivity (EBIC) measurements were applied. The conductivity in the QD plane decays within a distance of 10 microns. The capacitance transients are dominated by the local QD-plane transversal conductivity and by the free carrier transport, in the cap layer. The nonequilibrium free carrier created by electron beam excitation develop a potential barrier at macroscopic distances from the electrical contacts.

Photoluminescence energy and linewidth in GaN/AlN stackings of quantum dot planes

We analyze the room temperature photoluminescence properties of several multilayer stackings of GaN/AlN quantum dots. We report drastic differences of emission energies and linewidths between continuous wave and time-resolved photoluminescence experiments. In continuous wave experiments, the screening of internal electric fields by accumulation of e-h pairs in quantum dot planes induces a blue-shift as well as an unexpected narrowing of the emission line, when the laser intensity is increased. Under intense, pulsed excitation, in time-resolved photoluminescence, a substantial blue-shift is induced, due to the partial cancelation of the quantum confined Stark effect. When the system is again free to relax, we observe a time-dependent red-shift of the line, which maintains a fairly constant width. We attribute the observed behavior of energies and linewidths to the intricate contributions of the in-plane distribution of dot sizes and of the depth-dependent decrease of the degree of excitation of the different planes. We support our interpretations by the use of a model based on a self-consistent solution of the Schrödinger and Poisson equations within the envelope function approximation.

Electronic and Structural Properties of Interdiffused Self-Assembled Quantum Dots from Magneto-Photoluminescence

A set of self-assembled InAs/GaAs quantum dot (QD) samples annealed at various temperatures for 30 s was studied using magneto-photoluminescence up to 28 T. Blueshifts increasing with annealing temperature, due to Ga-As interchange at the QD-barrier interface, are correlated with a reduction in inhomogeneous broadening and a reduction in inter-sublevel spacing. These new sample properties allow us to obtain clear crossing patterns closely matched with Fock-Darwin diagrams where the field applied perpendicular to the QD plane lifts the state degeneracies. In the limit that in-plane electron and hole wavefunction extension is the same, the splitting of the p-shell with magnetic field is inversely proportional to the in-plane exciton reduced mass. We use this to obtain the evolution of the latter with intermixing, and compare with predictions of single-particle k*p calculations.

Zeeman energy and anomalous spin splitting in lateral GaAs quantum dots

The level splittings induced by a horizontal magnetic field in a parabolic two-dimensional quantum dot with spin-orbit interaction are obtained. Characteristic features induced by the spin-orbit coupling are the appearance of zero-field gaps as well as energy splittings that depend on the electronic state and the orientation of the magnetic field in the quantum-dot plane. It is suggested that these quantum-dot properties could be used to determine the Rashba and Dresselhaus spin-orbit intensities

Electronic states and vibration spectra of CdTe/ZnTe quantum dot superlattices

Electronic and vibrational states in CdTe/ZnTe quantum dot superlattices are studied using optical spectroscopy techniques (photoluminescence in a wide temperature range, IR reflection, and Raman scattering). The effect of the ZnTe barrier layer thickness on the luminescence spectra of the structures is discussed. The luminescence from electronically coupled islands is assumed to be due to spatially indirect excitons because of the specific features of the CdTe/ZnTe heterostructure band structure. A combination of quantum-dot vibrational modes, which has not been observed earlier, is detected in the Raman spectra. Analysis of the lattice IR reflection spectra shows that, in the case of large barrier thicknesses between the quantum-dot planes, elastic stresses are concentrated in the Zn1−xCdxTe layers, whereas in structures with lower barrier thicknesses the elastic-strain distribution exhibits a more complicated pattern.

Carrier-diffusion-limited remote optical addressing of single quantum dots

We present a scheme for remotely addressing single quantum dots (QDs) by means of near-field optical microscopy that simply makes use of the polarization of light. A structure containing self-assembled CdTe QDs is covered with a thin metal film presenting sub-wavelength holes. When the optical tip is positioned some distance away from a hole, surface plasmons in the metal coating are generated which, by turning the polarization plane of the excitation light, transfer the excitation towards a chosen hole and induce emission from the underlying dots. In addition, our procedure gives valuable insight into the diffusion of photo-excited carriers in the QD plane that can put limits to the addressing scheme.

Resonant tunneling theory of planar quantum dot structures

The ballistic transport in the semiconductor, planar, circular quantum dot structures is studied theoretically. The transmission probabilities show apparent resonant tunneling peaks, which correspond to energies of bound states in the dot. By use of structures with different angles between the inject and exit channels, the resonant peaks can be identified very effectively. The perpendicular magnetic field has obvious effect on the energies of bound states in the quantum dot, and thus the resonant peaks. The treatment of the boundary conditions simplifies the problem to the solution of a set of linear algebraic equations. The theoretical results in this paper can be used to design planar resonant tunneling devices, whose resonant peaks are adjustable by the angle between the inject and exit channels and the applied magnetic field. The resonant tunneling in the circular dot structures can also be used to study the bound states in the absence and presence of magnetic field.

Effect of a Charged Impurity on a Lithium Quantum Dot

A lithium quantum dot (QD) with a charged impurity is studied, where the charged impurity is located on the z-axis at a distance d from the lithium QD plane (the x–y plane). Using the method of few-body physics, the energy spectra of the low-lying states of the QD are obtained as a function of d. It is found that the energy spectra depend strongly on both the position and charge of the impurity. Furthermore, the impurity affects the electronic configurations of the QD, i.e. the spatial distribution of the electrons of the QD. Our calculations indicate that the change of d induces transitions in the ground states of the QD, which can be explained by dynamics and symmetry.

Barrier Li Quantum Dots in Magnetic Fields**The project supported by National Natural Science Foundation of China under Grant Nos. 90103028 and 10174098

The methods for the few-body system are introduced to investigate the states of the barrier Li quantum dots (QDs) in an arbitrary strength of magnetic field. The configuration, which consists of a positive ion located on the -axis at a distance d from the two-dimensional QD plane (the plane) and three electrons in the dot plane bound by the positive ion, is called a barrier Li center. The system, which consists of three electrons in the dot plane bound by the ion, is called a barrier Li QD. The dependence of energy of the state of the barrier Li QD on an external magnetic field B and the distance d is obtained. The angular momentum L of the ground states is found to jump not only with the variation of B but also with d.

High external differential efficiency and high optical gain of long-wavelength quantum dot diode laser

Long-wavelength (1.29 μm) lasers grown on GaAs and based on several planes of self-organized quantum dots in an external quantum well demonstrate significant improvement of the external differential efficiency (88%) and the characteristic temperature (150 K) . This is due to suppression of carrier pile-up in the waveguide region in combination with extended range of optical loss in which the ground-state lasing survives.

1.5 micron InAs quantum dot lasers based on metamorphic InGaAs/GaAs heterostructures.

ABSTRACT1.5 micron range emission has been realized using the InAs quantum dots embedded into the metamorphic InGaAs layer containing 20% of InAs grown by MBE on a GaAs substrate. Growth regimes were optimized to reduce significantly the density of dislocations propagating into the active layer from the lattice mismatched interface. 2 mm long InGaAs/InGaAlAs lasers with 10 planes of quantum dots in the active region showed threshold current density about 1.4 kA/cm2 with the external differential efficiency as high as 38%. Lasing wavelength depends on the optical loss being in the 1.44–1.49 micron range at room temperature. On increasing the temperature the wavelength reaches 1.515 micron at 85C while the threshold current characteristic temperature of 55–60K was estimated. High internal quantum efficiency (η>60%)and low internal losses (α=3–4 cm ) were realized. Maximum room temperature output power in pulsed regime as high as 5.5 W for 100 micron wide stripe was demonstrated. Using the same concept 1.3 micron InGaAs/InGaAlAs quantum well lasers were fabricated. The active region contained quantum wells with high (∼40%) indium content which was possible due to the intermediate InGaAs strain relaxation layer. 1 mm stripe lasers showed room temperature threshold current densities about 3.3 kA/cm (λ=1.29 micron) and 400 A/cm2 at 85K. Thus, the use of metamorphic InGaAs layers on GaAs substrate is a very promising approach for increasing the emission wavelength of GaAs based lasers.

1.5 micron InAs quantum dot lasers based on metamorphic InGaAs/GaAsheterostructures.

ABSTRACT1.5 micron range emission has been realized using the InAs quantum dots embedded into the metamorphic InGaAs layer containing 20% of InAs grown by MBE on a GaAs substrate. Growth regimes were optimized to reduce significantly the density of dislocations propagating into the active layer from the lattice mismatched interface. 2 mm long InGaAs/InGaAlAs lasers with 10 planes of quantum dots in the active region showed threshold current density about 1.4 kA/cm2 with the external differential efficiency as high as 38%. Lasing wavelength depends on the optical loss being in the 1.44–1.49 micron range at room temperature. On increasing the temperature the wavelength reaches 1.515 micron at 85C while the threshold current characteristic temperature of 55–60K was estimated. High internal quantum efficiency (η>60%) and low internal losses (α=3–4 cm-1 ) were realized. Maximum room temperature output power in pulsed regime as high as 5.5 W for 100 micron wide stripe was demonstrated. Using the same concept 1.3 micron InGaAs/InGaAlAs quantum well lasers were fabricated. The active region contained quantum wells with high (∼40%) indium content which was possible due to the intermediate InGaAs strain relaxation layer. 1 mm stripe lasers showed room temperature threshold current densities about 3.3 kA/cm2 (λ=1.29 micron) and 400 A/cm2 at 85K. Thus, the use of metamorphic InGaAs layers on GaAs substrate is a very promising approach for increasing the emission wavelength of GaAs based lasers.

Magnetic field effects on intersubband transitions in a quantum nanostructure

We report on our theoretical studies of the magnetic field effects on intersubband transitions in quantum dots (QDs) embedded in a quantum cascade structure subjected to a strong magnetic field applied perpendicular to the QD plane. The peaks of the emission spectra calculated for QDs containing a few interacting electrons indicate that there is a correlation between the non-monotonic behavior of the maximum value of the emission spectrum as a function of the applied magnetic field and the field dependence of the low-lying energy spectra of the dots.

Encoding of holographic grating and periodic nano-structure by femtosecond laser pulse

By fully utilizing distinct features of femtosecond (FS) pulses such as ultra-short time domain and good coherence, we have successfully encoded micro-gratings and multi-dimensional periodic nano-structures either on or beneath surfaces of versatile materials including dielectrics, semiconductors, metals and plastics. In the setup for the encoding, a regenerative amplified pulse from a mode locked Ti:sapphire laser was split into two beams, then overlapped on the top surface or in the interior of the materials to give a ∼50 μm spot. The optical paths of the two beams should be adjusted precisely within ∼30 μm to induce the interference. Use of the third harmonic generation (THG) from air made the adjustment possible not restricted from colliding angles. Surface relief type gratings due to laser ablation and refractive index type gratings resulted from structural changes in silica glasses, embedded gratings in diamond, gratings capable of light coupling and decoupling to a LiNbO 3 wave guide, and two-dimensional nano-structures in silica glass are demonstrated as examples. We expect the present technique to open up a frontier of diffractive optics with the formation of periodic nano-structures applicable to emerging devices such as planar wave guides, quantum dots and photonic crystals.

Effect of the Electron-Phonon Coupling on Barrier D- Quantum Dots in Magnetic Fields**The project supported by National Natural Science Foundation of China under Grant No. 19975013 and Backbone Teacher Foundationof Universities in China

The influence of the electron-phonon coupling on the energy of low-lying states of the barrier D- center, which consists of a positive ion located on the z-axis at a distance from the two-dimensional quantum dot plane and two electrons in the dot plane bound by the ion, is investigated at arbitrary strength of magnetic field by making use of the method of few-body physics. Discontinuous ground-state energy transitions induced by the magnetic field are reported. The dependence of the binding energy of the D- ground state on the quantum dot radius is obtained. A considerable enhancement of the binding is found for the D- ground state, which results from the confinement of electrons and electron-phonon coupling.

Optical Properties of InAs/InP Planar Quantum Dot Microcavities

ABSTRACTPlanar InAs/InP quantum dot microcavities using multi-layer SiO2/Ta2O5 Bragg reflectors have been studied in emission. The spectra exhibit collection optics-limited cavity linewidths of ∼1meV with the occasional ∼200μeV single-dot emission. Measurements as a function of incident power show quantum dot saturation behavior, with transfer of oscillator strength to the wetting layer outside the cavity stop band. Saturation behavior at fixed pump power is also observed as a function of decreasing temperature. Dispersion measurements as a function of emission angle show polarization splitting in qualitative agreement with theory.